1. Field of the Invention
The present invention relates to a standard wave receiver that receives a standard wave and presents clock time, and a time code decoding method of decoding a time code signal superimposed on the standard wave.
A standard wave providing the Japanese Standard Time is always transmitted by long waves of 40 kHz and 60 kHz from two locations in Japan, the Kyushu Long Wave Station and the Fukushima Long Wave Station that are operated and managed by the Communication Research Laboratory. A carrier wave of such a standard wave is subjected to amplitude modulation according to a time code signal (hereinafter also referred to as TCO signal) that is generated at a bit rate of 1 bit/second. In the time code signal, one frame consisting of 60 bits is repeated every one minute. Time information including year, month, day, hour and minute is stored in each frame in a notation form of a Binary Coded Decimal code (BCD) (see FIG. 1).
A code of one bit forming the time code signal is any one of three codes, namely, a binary “1”. code indicating binary “1”, a binary “0” code indicating binary “0”, and a marker code (for convenience, indicated by “2” or “M”) that is a synchronizing signal for indicating a partition of time information. In that sense, it should be noted that the term “bit” used in this specification is different from a usual example of the term. The three codes are distinguished according to a difference of an H width in a square pulse (see FIG. 2).
It is well known that, in actual reception of such a standard wave, a problem occurs in precise decoding of the time code signal. For example, a noise signal is superimposed on a received wave because of sferics noise or noise caused by automobiles or apparatuses such as home appliances. In such cases, a starting point of a rising edge of a square pulse of the time code signal cannot be detected precisely. Thus, bit synchronization is inaccurate. Under a reception environment with a low field intensity, a square pulse is distorted to make it difficult to decode a code precisely.
2. Description of the Related Art
A technique disclosed in JP-A-2003-215277 makes it possible to overcome such a problem by additional processing for sampling an integral value of a time code signal pulse, which is generated from a standard wave, every predetermined time to distinguish a code.
However, a basic approach of such a method simply realizes precise decoding of respective square pulses by calculating an integral value of one pulse waveform. Therefore, under a reception environment with extremely inferior noise intensity, field intensity, or the like, even decoding of a waveform cannot be performed, to say nothing of the presence or absence of a decoding error.